The testis is an immunologically privileged site. In vivo and in vitro studies suggest that the germ cells are responsible for this immune privilege. Our hypothesis is that the germ cells are producing some factor which is present in sufficient concentrations throughout the testis to prevent a normal first set immune rejection response to foreign antigens. This testicular immunosuppressive factor (TIF) presumably exists to prevent the autodestruction of the germ cells, which are known to have immunogenic surface determinants. Our proposal will: (1) show in vivo whether germ cells are required for the testis to be a privileged site; (2) identify the fraction of the testis which contains this immunosuppressive factor and isolate this factor by in vivo immunologic testing and (3) characterize this factor biochemically.